PHYSICAL THEORY OF ASCENT OF SAP

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Theories of Water Ascent in Plants

The movement of water from the roots to the leaves in tall plants, often against gravity, has been a subject of extensive research. Several theories have been proposed to explain this phenomenon, each contributing to our current understanding.

1. Physical Force Theory (General Concept)

This is a broad category encompassing various early ideas that attributed water movement to simple physical forces, often without a detailed molecular or cellular mechanism. These theories generally focused on forces like root pressure, simple suction, or the idea of a pushing force from the roots. While some of these "physical forces" like root pressure do play a minor role, they are insufficient to explain water transport over long distances, especially in tall trees. Later, the more refined "cohesion-tension theory" also identifies a physical force (transpiration pull) as the primary driver, but distinguishes itself by detailing the specific molecular properties of water that enable this.

2. Atmospheric Pressure Theory

This theory proposed that atmospheric pressure pushes water up the xylem vessels, similar to how a vacuum pump works. It was suggested that transpiration (water loss from leaves) creates a partial vacuum in the xylem, and then atmospheric pressure acting on the water in the soil around the roots pushes the water column upwards.

  • Mechanism: Transpiration was thought to create a negative pressure (suction) in the leaves, which would then be transmitted down to the roots. The external atmospheric pressure, acting on the water in the root, would then force the water column up the xylem vessels.
  • Limitations:
    • Atmospheric pressure at sea level can only support a column of water up to approximately 10.3 meters. This means it cannot explain water transport in trees taller than this height.
    • This theory doesn't account for the intrinsic properties of water and the plant's vascular system that allow for continuous upward movement.

3. Imbibition Theory

Imbibition is a type of diffusion where water is absorbed by solid particles (colloids) causing them to swell. This theory suggested that the hydrophilic (water-attracting) components of the cell walls of xylem vessels, particularly cellulose and pectin, would imbibe water and then pull it upwards.

  • Mechanism: The walls of the xylem elements and other plant cells contain colloidal substances that have a strong affinity for water. It was proposed that this imbibitional force could be strong enough to draw water up the xylem.
  • Limitations:
    • Imbibitional forces are significant for short distances and in specific contexts (like seed germination or early seedling growth), but they are too weak to lift water to the tops of tall trees.
    • The primary pathway for long-distance water transport is the lumen (empty space) of the xylem vessels, not through the cell walls themselves. Water flows through the continuous column within the vessels, not by being absorbed and released by the walls.
    • The rate of water movement by imbibition is much slower than the observed rates of sap ascent.

4. Capillary Theory of Bohm

This theory, prominently advocated by Bohm, attributes the ascent of sap partly to capillary action within the narrow xylem vessels. Capillarity is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity.

  • Mechanism: Capillary action relies on two main forces:
    1. Adhesion: The attractive force between water molecules and the hydrophilic walls of the xylem vessels.
    2. Surface Tension: The cohesive force between water molecules themselves, which creates a "skin" at the water surface. These forces together cause water to rise in narrow tubes. The narrower the tube, the higher the water can rise due to capillarity.
  • Limitations:
    • While capillary action occurs in the xylem and contributes to water distribution within tissues, it alone cannot explain the ascent of sap to great heights. Even the narrowest xylem vessels (around 20-200 micrometers in diameter) can only lift water to a few meters (e.g., in a typical xylem vessel of 50 µm diameter, water would rise only about 0.6 meters due to capillarity).
    • The continuous column of water is often subject to breaks due to cavitation (formation of air bubbles), which would interrupt or stop capillary rise if it were the sole mechanism.

5. Cohesion-Tension Theory (Dixon and Joly, 1894)

This is the most widely accepted and comprehensive theory explaining the long-distance transport of water in plants. It postulates that water is pulled upwards by a negative pressure (tension) generated by transpiration from the leaves, and this pull is transmitted down the continuous column of water due to the cohesive properties of water molecules.

  • Key Components:

    1. Transpiration Pull (Tension):

      • Water evaporates from the surfaces of mesophyll cells in the leaves into the intercellular spaces and then diffuses out through stomata into the atmosphere.
      • This loss of water creates a negative pressure or tension (less than atmospheric pressure) in the mesophyll cells.
      • This tension extends to the continuous water columns in the xylem vessels in the leaf veins.

    2. Cohesion of Water Molecules:

      • Water molecules are polar and form hydrogen bonds with each other, resulting in a strong mutual attraction. This property is called cohesion.
      • Due to cohesion, water molecules stick together, forming an unbroken, continuous column of water throughout the xylem vessels, from the roots to the leaves. This column resists being pulled apart under tension.

    3. Adhesion to Xylem Walls:

      • Water molecules also adhere strongly to the hydrophilic (water-attracting) lignin and cellulose walls of the xylem vessels. This property is called adhesion.
      • Adhesion helps maintain the continuous water column by preventing it from pulling away from the xylem walls, especially under tension, and helps resist the inward pull of the xylem walls due to tension.

    4. Continuous Water Column:

      • The combined forces of cohesion and adhesion ensure that the water in the xylem forms a continuous, unbroken column, extending from the root hairs, through the root cortex, into the xylem of the stem, and finally into the xylem of the leaves and up to the mesophyll cells.
      • This column acts like a rope that can be pulled from above.

  • Overall Mechanism:

    • As water evaporates from the leaves (transpiration), it creates a strong negative pressure (tension or "pull") in the xylem of the leaves.
    • This tension is transmitted downwards through the continuous, cohesive water column in the xylem vessels, all the way to the roots.
    • The tension pulls water from the root xylem into the stem xylem, and then water from the soil into the root xylem (replacing the water lost by transpiration).
    • The narrowness of the xylem vessels (capillarity) also plays a supportive role by increasing the surface area for adhesion and enhancing the strength of the water column against breakage.

  • Evidence and Strengths:

    • This theory is able to explain water transport to the tallest trees (over 100 meters).
    • Measurements have confirmed the existence of tension in xylem sap.
    • It logically integrates transpiration, water's physical properties, and the plant's vascular structure.
    • Xylem vessels are well-adapted for this role, with lignified walls to withstand negative pressure without collapsing.

In summary, while early theories like atmospheric pressure and capillarity offered partial explanations, the Cohesion-Tension Theory, driven by transpiration pull and supported by the unique cohesive and adhesive properties of water, provides the most comprehensive and widely accepted model for water ascent in plants. Imbibition and root pressure play minor or localized roles.

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